Comprehensive Performance Evaluation and Life Cycle Analysis of a Pebble Bed-Integrated Forced Convection Solar Dryer for Agro-Processing

Abstract

This paper presents a rigorous thermal, exergetic, environmental and economic performance evaluation of a singleglazed, single-pass, forced-convection indirect solar dryer integrated with a packed-bed sensible heat storage (SHS) unit. Experimental investigations were performed in Coimbatore, India (11.0183°N, 76.9725°E) using a 2 m² corrugated galvanised iron absorber plate tilted at 11° and a 110 kg pebble packed bed (30–50 mm diameter, porosity φ = 0.85) acting as the SHS medium. Eight air mass flow rates ranging from 0.0141 kg/s to 0.0872 kg/s were evaluated. Experimental results confirm that the SHS unit extends collector operation by 2 h post-sunset, maintaining air temperatures 6–8 °C above ambient. Bitter gourd (Momordica charantia) slices (91.75 % w.b. initial moisture) and curry leaves (Murraya koenigii, 67.3 % w.b.) were dried to final safe moisture levels. At the optimal flow rate of 0.0636 kg/s, bitter gourd drying was completed within 7 h with a maximum dryer thermal efficiency of 19.12 %, an SMER of 0.225 kg/kWh and a minimum specific energy consumption (SEC) of 4.440 kWh/kg water. Thin-layer drying kinetic analysis identified the Two-Term model (R² = 0.998646) for bitter gourd and the Modified Henderson and Pabis model (R² = 0.999337) for curry leaves as the best mathematical representations. Life cycle assessment revealed an embodied energy of 1109.3 kWh, an energy payback time (EPBT) of 2.21 years, a financial payback period of 2.71 years and a net lifetime CO₂ mitigation of 13.07 tonnes over a 15-year lifespan